Tony Revolori, who has recently appeared in The 5th Wave and Dope, and starred in The Grand Budapest Hotel as the lovable Zero, was already confirmed as appearing in Spider-Man: Homecoming, but his role was unknown. At the Marvel Studios panel at SDCC, however, Revolori's character was confirmed in the footage shown.
The footage was an entire scene, set at Midtown High, where Peter Parker, played by Tom Holland, attends school. Other previously confirmed castmates Jacob Batalon, Laura Harrier, and Zendaya also appeared in the trailer. In the scene, we saw a typical day of high school for Peter, as he went from class to class, ate lunch in the cafeteria, and dealt with the problems of being a high schooler who is also a superhero.
In his science class, Peter is goofing off, watching a video on youtube of Spider-Man teaming up with Iron Man and War Machine to take down Giant Man at the airport in Germany, as seen in Captain America: Civil War. The teacher calls first on Flash Thompson to answer a question - and that's when Revolori speaks up. Yes, he's Flash, a traditionally (in the comics) white character whose only real defining characteristic is that he's a jerk/bully to Peter, yet ironically idolizes Spider-Man.
Comicbook.com confirmed independently that is his role in the film. So that's one more of Peter's classmates that we know for sure is playing a character from the comics! Whether that leads to any of Flash's more advanced stories (he eventually joins the Army, loses his legs, and is bonded with the Venom Symbiote as 'Agent Venom' - all things several years off from happening here) will need another decade or so to play out, of course.
Spider-Man: Homecoming is filming now for a July 7, 2017 release.